The pictures you've already attached show up as a list in your message when you add more than one. To make them visible, do the following:

When you are in that small attachment window attaching the pictures, click the attachment you uploaded, and it will open in a new window. Copy that URL in the new window, and go back to your message window. Click the "insert image" button in your message (looks like this: ), and insert that URL. Go back to the small attachment window, click the second attachment, and do the same.